The present invention relates to a method for producing a halftone plate from an original picture having a continuous tone, and more particularly relates to a method for producing a halftone plate composed of halftone dots which can be effectively processed by the dot etching.
In a conventional picture reproducing machine such as a color scanner, a halftone plate is produced from an original picture having a continuous tone by exposing a lithographic recording film via a halftone contact screen overlapped thereon by using a light beam modulated by a picture signal picked up from the original picture. Alternatively, the original picture and the halftone contact screen are scanned separately in the synchronous relation to obtain output signals and then the output signals are treated electrically to prepare a halftone plate.
However, in such methods wherein the halftone contact screen is used, there are disadvantages such as high cost, unevenness of halftone dots by a poor contact between the halftone contact screen and the film, a large exposure light source, and an inevitable complicated and large machine.
Other methods wherein the halftone dots are produced electrically, have been developed. For example, a halftone plate is produced on a recording film by means of an exposure light beam whose width and central position are controlled depending on the picture signal, or by means of an exposure light beam consisting of a line of light spots, each being adapted to be turned on or off independently according to the picture signal.
In the latter method, as disclosed in Japanese Patent Publication No. 52-33523, the peripheral and the central parts of the halftone dot are exposed by substantially the same luminous energy, and thus the obtained halftone dot includes extremely less fringe part, i.e. a so-called very hard halftone dot is obtained. It is sometimes inconvenient for the original film for plate making.
In this method, the desired halftone plate is not always obtained, and it often happens that a part or the entire of the halftone plate is to be corrected. On such an occasion, the halftone dots are diminished or enlarged by the dot etching, that is, etching the silver deposit composing the halftone dots on the halftone film, partly or throughout its surface.
However, in the hard halftone dot as described above, the central and the peripheral parts are exposed by substantially the same luminous energy, and hence the same amount of silver metal is deposited over the halftone film. Therefore, such a hard halftone dot can not be diminished gradually from the peripheral part to the center, and the silver deposit is reduced to the same amount in the central and the peripheral parts instead, with the result of a useless halftone plate.
In order to remove this disadvantage, another method has been proposed, as disclosed in the U.S. Pat. No. 4,025,189. In this method, an addition or subtraction value of two values of a picture signal picked up from the original picture and a signal generated by a screen pattern generator, is used. That is, the two addition or subtraction values for controlling the luminous energies of the adjacent two of light beams arranged in parallel with one another, are compared in numerical order.
Then, when the two addition or subtraction values for the adjacent two light beams satisfy the same condition such as exposure or non-exposure, at the same time, the luminous energy of the light beam of the latter number is controlled depending on its addition or subtraction value, i.e. the exposure or the non-exposure condition. Meanwhile, assuming that the two addition or subtraction values for the adjacent two light beams are a and b, when the values a and b are different, a calculation ##EQU1## is carried out, and the light beam of the latter number is controlled to the intermediate luminous energy between the exposure and the non-exposure conditions depending on the calculation result C.
However, this method requires not only an addition or subtraction circuit but an absolute-value circuit and a dividing circuit. Hence, this method involves a complicated machine and the division in the digital manner requires rather long time, which is inconvenient for the rapid operation which is getting quicker and quicker in future, required to the color scanner and the like.